A removable adhesive is an adhesive that can be easily removed without leaving stains or debris from a substrate where it has been adhered.
The pressure sensitive adhesive sheet applied to the removable adhesive on one side of the substrate can be printed by graphic printing of the substrate using inkjet printer or air-brush printer and then used for POP display, plane advertisement on bus or taxi or automobile wrapping or advertisement on window or floor.
Such pressure sensitive adhesive sheet is used not only indoor but also outdoor, so it needs to be designed to have low glass transition temperature (Tg, at least −40° C.) in order to demonstrate satisfactory workability and adhesiveness at low temperature. Considering the surface where it is used, that is it can be adhered on irregular surface or indented surface, and considering the duration, for example, it might needs to be adhered for months or 2 years at most, the adhesive for such pressure sensitive adhesive sheet needs to have excellent adhesive power, dimensional stability to maintain the original size of the pressure sensitive adhesive sheet, and weatherability so as to maintain its physical properties for a long term.
Particularly, it is very important for a removable adhesive to have excellent removability without leaving residues after being adhered once. In most cases, when removability and dimensional stability are increased, adhesive power is decreased. So, it is generally understood that a removable adhesive has weaker adhesive power than a permanent adhesive leaving residues when it is removed.
To increase physical properties of a removable adhesive, it is necessary to regulate the molecular structure such as compositions, cross-linking density, molecular weight and molecular weight distribution of the adhesive. For example, when a cross-linking agent is properly regulated, removability of a removable adhesive can be adjusted.
U.S. Pat. No. 5,240,989 describes a removable acrylic pressure sensitive adhesive having glass transition temperature (Tg) of −50-−55° C. prepared by using a multifunctional diacrylate cross-linking agent having different functional groups including hexanedioldiacrylate. According to this description, a proper amount of gel is necessarily included in the resin in order for the adhesive to have removability. At this time, the preferable content of such cross-linking agent is 0.1-0.3 weight part, according to the patent.
U.S. Pat. No. 6,586,510 describes an acrylic pressure sensitive adhesive with modified removability prepared by using a multifunctional cross-linking monomer such as allylmethacrylate, diallylmalate or ethylene glycol diacrylate at the concentration of 0.01-1 weight part.
US Patent Publication No. 2006-0036027 (2006.02.16.) describes a removable adhesive having strong resistance against a plasticizer prepared by using a cross-linking agent such as diallylmalate, allylacrylate or allylmethacrylate at the concentration of 0.3-0.6 weight part.
However, the mentioned prior arts cannot provide satisfactory removability and adhesiveness at room temperature and under aging.